386 ORGANS OF THE SENSES. 



It must not, however, be supposed that the boundary be- 

 tween these two classes of sensations is strictly defined ; on 

 the contrary, a certain confusion exists between them, pro- 

 duced by the infinite number of transitionary sensations; 

 thus, for instance, one impression will be perceived for two 

 or three reflex phenomena which will pass unperceived ; and 

 thus the stomach, which generally has little sensibility, 

 sometimes in a physiological state becomes extremely sensi- 

 tive to the presence of food or of foreign bodies. 



Now that we know both the character of the phenomena 

 of sensation, and the surfaces which form their starting-point, 

 we will take up the study of general and special sensations 

 experienced at each of these surfaces. , 



I. GENERAL SENSATIONS. 



THE general sensations are very widely diffused. Many 

 surfaces give rise to those kinds of sensation only, which give 

 no indication as to the character or qualities of the bodies 

 which make the impression ; but show their influence only by 

 impressions which it is difficult to define, such as pleasure or 

 pain, or even by effects which are still less easily described, 

 and which belong in a great measure to the class of reflex 

 phenomena, such as tickling, for instance. 



The mucous surfaces in general yield only extremely vague 

 sensations. The digestive mucous tissue gives little indica- 

 tion as to the form, the temperature, and other properties of 

 the bodies brought in contact with it, excepting only its 

 upper portion (the mouth) where it exhibits a special arrange- 

 ment, by means of which it becomes the seat of a special 

 sensation, and constitutes the organ of a sense (taste) which 

 we shall presently study. In cases of fistula of the stomach 

 or intestines, however, various substances have been intro- 

 duced into these tubes, and their internal surface touched 

 with different excitants, without the patient experiencing any 

 distinct perception, or any sensation resembling those which 

 we shall examine under the name of touch. 



The vague sensation which warns us of the need of food 

 appears to be a gastric sensation : the seat of hunger is sup- 

 posed to be in the upper part of the digestive tube; we have 

 already seen, however, that this sensation is connected with 

 a general feeling of discomfort, and that it is a call from the 

 impoverished blood for nourishment. The ground of this 



